1. Field of the Invention
The present invention relates to cellular telephone and personal communication services (PCS) systems using code division multiple access (CDMA) techniques. More specifically, the present invention relates to a novel and improved method for determining the required amount of forward link channel powers for a CDMA cellular or PCS system in which the forward link channels are orthogonally multiplexed, so that forward link interference in the system is kept to a minimum by using only the required amount of power for each channel.
2. Description of the Prior Art
In a cellular system, each base station transmitter is intended to provide a reliable and sufficiently strong signal level to mobile receivers in a given area (cell), but not to mobile receivers in other cells. However, it is physically impossible to prevent the signal from propagating to adjacent cells, thereby causing interference to the base station signals in those areas. This other-cell forward link interference imposes a fundamental limitation on the performance of the system with regards to capacity, since the maximum number of mobile users is inversely proportional to the amount of interference from any channel.
For the synchronization (sync) channel of a CDMA cellular system, for example, the power delivered by the base station power amplifier, denoted as P2, is subject to various system losses and gains, including propagation loss and transmitter and receiver antenna gains, before reaching the input of a mobile receiver. Thus the power at the in put of a mobile receiver is P2/L, where L denotes the net loss on the link. Since the sync channel signal is intended to be received by any mobile user in the cell, L is the net loss at a distance equal to the call radius, i.e. the distance to the cell edge. In order to ensure satisfactory operation of the sync channel, the ratio of received sync channel power to the sum of noise and interference power at the receiver must exceed a certain amount as per the following:
g2P2/L÷(Nm+IT)xe2x89xa7xcfx812xe2x80x83xe2x80x83(1)
where Nm i s the receiver thermal noise power, IT is the total interference power at mobile receiver input, xcfx812 is the required bit energy-to-noise density ratio for the sync channel, and g2 is the spread spectrum processing gain for the sync channel. The total interference power includes interference from other cells and interference from the same cell as per the following:
IT=Iother+Isamexe2x80x83xe2x80x83(2)
The primary component of the interference power at a mobile receiver located at the edge of the cell is due to base station signals from adjacent cells, since in a CDMA system each cell uses the same frequency band. In lieu of a measurement, it is common in CDMA cellular engineering to assume that each base station emits the same amount of total power. It is also common to estimate the amount of other-cell interference at the cell edge as a factor, Kother times the total forward link power received from the mobile""s own base station as per the following:
Iother=KotherPt/Lxe2x80x83xe2x80x83(3)
where Pt denotes the total forward link power of a base station, which is the sum of the powers of the orthogonally multiplexed forward link channels. Studies have shown that the worst case value of Kother for a hexagonal pattern of cells is on the order of 1.8 or 2.5 dB.
In a CDMA cellular system, ideally there is no same-cell interference on the forward link because the different channels are orthogonally multiplexed. However, same-cell interference can exist on the forward link due to multi-path receptions of the total forward link waveform at a particular location. In lieu of a measurement, it is convenient to model the amount of same-cell interference at the cell edge as a factor, Ksame, times the total received forward link power as per the following:
Isame=KsamePt/Lxe2x80x83xe2x80x83(4)
The amount of forward link multi-path interference varies from place to place. It is common in CDMA cellular engineering to conservatively estimate that the total same-cell interference power from one or more multi-path receptions is equal to the power of the main or direct path, so that the value Ksame=1 is often used. The other-cell interference and the same-cell interference have the same form and it is therefore possible to characterize the total interference using a factor, Kf=Ksame=Kother, to express the total interference as a multiple of the received power as per the following:
IT=KfPt/Lxe2x80x83xe2x80x83(5)
It is clear that if the interference power increases, the base station power must be increased in order to maintain the same value of the power ratio. However, increasing power at one base station results in more interference to mobile users in adjacent cells.
A preferred method, which is in accordance with the cellular concept, is to minimize forward link interference by using as little base station transmitter power as possible while maintaining reliable signal levels at the intended receiver locations. However, in order to implement such a method, it is necessary to use some method for adapting the transmitter power of each forward link channel to the interference situation in such a way as to use only the required amount of power for that channel.
For example, U.S. Pat. No. 5,103,459 describes a system for implementing a CDMA cellular telephone system in which the forward link transmissions from the base station to the several mobile receivers are one-to-many transmissions of a single waveform comprised of the orthogonally combined (multiplexed) transmissions to particular mobile users, as well as signaling channels broadcast to all active mobile users and to areas of the cell or cellular sector in which potential mobile users may be located. The system described above subsequently embodied in the industry standard IS-95, features four types of forward link channels: (1) a pilot channel consisting of a spread spectrum pseudo noise (PN) coded waveform with no modulation, which is a signaling channel designed to facilitate acquisition of the base station signal by a mobile user; (2) a synchronization channel consisting of the same PN coded waveform as the pilot channel but modulated with timing and other data needed as a signaling channel to set up the mobile unit for further activity in the system; (3) one or more paging channels that are signaling channels available to the base station for transmitting various messages to particular mobile units by modulating the same PN coded waveform used as the pilot channel; and (4) one or more traffic channels that carry digital voice data modulated onto the PN coded waveform. However, such system does not describe implementation of gain control or a method for determining the values of the channel powers that dictate what the channel gains should be.
Furthermore, with regards to the non-full duplex forward link signaling channels of a CDMA cellular system, including the pilot, sync, and paging channels, there is no provision built into the system for adaptively controlling transmitted power based on feedback from mobile receivers. Since it is necessary to increase or decrease signaling channel powers as interference increases or decreases, there arises the need for an auxiliary method for adjusting the power in the forward link signaling channels quickly and without consuming additional communication resources.
A common method calls for CDMA operators to assign the pilot channel and other signaling channel powers fixed percentages of the total forward link power, as suggested in the book CDMA: Principles of Spread Spectrum Communication, by A. J. Viterbi, and in the paper xe2x80x9con the Capacity of a Cellular CDMA System,xe2x80x9d by K. S. Gilhousen et al. Using this method, the signaling channel powers are made proportional to the number of active traffic channels, since total power increases with the number of active channels. In this manner, the signaling channel powers are automatically increased when there is more interference, an adaptation that is needed to combat the increased interference. However, it can be shown that the optimum values of the percentages of total power for the signaling channels, which ensure the minimum amount of power is used, are not fixed but vary adaptively as a function of cell traffic and other parameters.
Therefore, the present invention is directed to a method, in connection with a base station transmitter, for adaptively determining the CDMA forward link channel power requirements to further determine the amount of power required for each channel in the CDMA system, to thereby control the power to minimize forward link interference.
More specifically, the present invention is directed to a method of adaptively determining the optimal amount of power for every forward link channel in a CDMA system using orthogonally multiplexed channels. According to the method of the present invention, a base station transmitter is provided a system of equations in which forward link energy-to-noise density requirements are modeled for every signaling channel of the CDMA system and for a traffic channel directed to a cell edge of the CDMA system. The equations are then solved to determine the respective channel power for each of the signaling channels and the traffic channel to thereby minimize forward link interference by using only the required amount of power for each channel, and the necessary channel gain is then determined for each of the signaling channels and the traffic channel by utilizing the solved system of equations. This determination is made, and the powers changed, on a frequently period basis in order to maintain the correct amounts of power as conditions change.
The system of equations solved in the base station transmitter considers the following variables: desired energy-to-noise density ratio values for each channel type, spread spectrum processing gains for each channel type, net loss experienced by the base station transmitter when processing from an amplifier output thereof to an input terminal of a mobile receiver at the respective cell edge, thermal noise power at the mobile receiver, the number of active CDMA paging channels, an average forward link traffic channel voice activity factor, a forward link same-cell interference factor, the number of active traffic channels, a forward link power control factor, a forward link other-cell interference factor at the cell edge and total forward link traffic channel power.